The present invention relates to coupling devices, and particularly to such devices for coupling (sometimes called "launching") a free space laser beam into an optical fibre.
The coupling or launching of a laser beam into a single mode optical fibre is problematic because of the small fibre core diameter (around 5 microns). In order to have high coupling efficiency, the laser beam must be focussed to a spot with a diameter close to that of the fibre core. Not only high precision, but also high stability is required against mechanical drifts because of temperature cycles, laser warmup and cool down, strain release at mechanical adjustment points, and the like.
Mechanically adjusted systems are usually based on a tilting unit which is mounted on the laser front face and holds the end of the optical fibre. The tilt is applied simultaneously to the fibre and to the focussing lens. No translation adjustments are usually provided because the laser beam is sufficiently concentric with the mechanical fixing points on the laser front face. However, replacement of a laser is problematic because the front face may not be sufficiently perpendicular for this purpose to the beam, and therefore may require inconvenient adjustments to provide a good coupling (launching) efficiency. This may be avoided be equipping the laser with prealigned optics and a high precision optical plug, and the fibre with a high precision connector. However, such an arrangement is very expensive. Moreover, temperature changes (e.g., because of laser warmup and cool down) may result in mechanical drifts, thereby requiring further adjustments.
According to another coupling arrangement, the fibre end may be moved in front of the fixed lens, but this requires a set of at least two micrometric translation stages with submicron resolution; moreover, long term stability is also difficult to achieve.
Another technique for attaining the required high precision and high stability is to use an active positioning system in which the position of the optical fibre with respect to the laser beam is continuously detected and corrected by motors where necessary. Such active systems, however, are very expensive.